1. Field of the Invention
The present invention relates to a power feeding circuit, in particular, a power feeding circuit including: a battery; a load operated by receiving a normal current from the battery in a normal mode, and operated by receiving a dark current smaller than the normal current from the battery in a sleep mode; and a semiconductor switch interposed between the battery and the load.
2. Description of the Related Art
Conventionally, in a vehicle, electricity is supplied to a load via an electric wire from a power source such as a battery or an alternator. Such an electric wire is provided with a protective fuse. When the electric wire is short-circuited and excessive current over permissible current flows, the protective fuse is melted to stop the power supply and protects the electric wire before the electric wire is melted.
However, there is a problem that such a protective fuse is large, and space-consuming, and has high heating value. Therefore, it is thought to provide a semiconductor fuse composed of a semiconductor switch provided on an electric wire, and a controller for switching the semiconductor switch, for example, switching off the semiconductor switch when an excessive current is detected by such as a current sensor.
Incidentally, it is necessary to constantly supply electric power to an electronic device (for example, ECU) as a load mounted on a vehicle even in a sleep mode in which an ignition switch is off and a vehicle is stopped for maintaining clock function, holding memory or the like. Therefore, when the semiconductor fuse is installed, it is necessary to maintain the semiconductor switch on even in the sleep mode. Therefore, it is necessary to get a peripheral circuit such as above-described current sensor and the controller to work normally. Accordingly, power consumption of the peripheral circuit and the controller in the sleep mode cannot be reduced, and there is a fear that the battery may run out.
Further, as an electric power supplying technique to the electronic device in the sleep mode, a dark current measuring circuit for a vehicle shown in FIG. 12 is proposed (Patent Document 1). As shown in FIG. 12, a dark current measuring circuit for a vehicle 100 includes: a battery 101; an ECU 102 as a load operated by receiving power supply from the battery 101; a relay 103 interposed between the battery 101 and the ECU 102; a current measuring circuit 104 connected in parallel to the relay 103; and a control unit 105 connected to the current measuring circuit 104. The current measuring circuit 104 is composed of a current sensor 104a as a current detecting device, and a semiconductor switch 104b connected in series to the current sensor 104a. 
In a normal mode, the control unit 105 switches on the relay 103 and switches off the semiconductor switch 104b, thereby an electric current is supplied to the ECU 102 via the relay 103. In a sleep mode, the control unit 105 switches off the relay 103 and switches on the semiconductor switch 104b, thereby the electric current is supplied to the ECU 102 via the current measuring circuit 104. Therefore, in the normal mode, the current sensor 104a does not measure the electric current, and in the sleep mode, the current sensor 104a measures the electric current (dark current). Because a value of the dark current is smaller than the current in the normal mode, a measuring range of the current sensor 104a can be narrower than when the current supplied to the relay 103 is measured.
According to the dark current measuring circuit for a vehicle 100, in the sleep mode, the electric current is supplied to the ECU 102 via the current measuring circuit 104. Therefore, even in the sleep mode, it is necessary to activate the current sensor 104a, and switch on the semiconductor switch 104b. Therefore, even in the sleep mode, there is a first problem that the battery may run out due to the power consumption of the current measuring circuit 104.
Further, it is necessary for a load such as a motor sliding a reclining seat or a slide seat to supply electric power corresponding to operations of a reclining switch or a slide switch even in a sleep mode in which a vehicle is stopped and an ignition switch is off. Therefore, when the semiconductor fuse is provided, it is necessary to maintain the semiconductor switch on even in the sleep mode, and to get the peripheral circuit such as above-described current sensor and the controller to work normally. Accordingly, power consumption of the peripheral circuit and the controller in the sleep mode cannot be reduced, and there is a fear that the battery may run out.
Therefore, it is thought to provide a power feeding circuit 106 as shown in FIG. 13 for saving electric power by switching the semiconductor switch on only when the reclining switch or a slide switch is turned on. As shown in FIG. 13, the power feeding circuit 106 includes: a battery 101 as a power source; a reclining seat motor 102a and a slide motor 102b as a load operated by receiving the power supply from the battery 101; and a group of semiconductor switches 107 respectively provided on both ends of the reclining seat motor 102a and the slide motor 102b. 
Each of the group of semiconductor switches 107 is composed of a pair of semiconductor switches 107a connected in series between the battery 101 and a ground. A connecting point between the pair of semiconductor switches 107a is connected to the end of reclining seat motor 102a or the slide motor 102b. Thereby, a direction of the electric current flowing into the reclining seat motor 102a or the slide motor 102b is controlled by the semiconductor switch 107a. 
The power feeding circuit 106 further includes: a reclining seat switch 106a operated when the reclining seat is driven; a slide switch 106b operated when the seat is slid; and a controller 108 for switching the semiconductor switch 107a on/off corresponding to the operation of the reclining seat switch 106a or the slide switch 106b. 
According to the conventional power feeding circuit 106, four semiconductor switches 107a are needed with regard to the reclining seat motor 102a and the slide motor 102b. Resultingly, it is difficult to downsize or save weight of a relay block or a junction block on which a substrate to be controlled is mounted. Further, two power lines L1, and two signal lines L2 for connecting the controller 108 to the reclining seat switch 106a and the slide switch 106b are needed with regard to the reclining seat motor 102a and the slide motor 102b. Therefore, there is a second problem that a wiring harness in a vehicle becomes enlarged.
Further, as described above, the dark current in the sleep mode is greatly smaller than the current in the normal mode when the ignition is on. However, when a vehicle is stopped for a long time, for example, shipment of cargo, there is a fear that due to even this small dark current, the battery may run out.
Therefore, in order to eliminate the dark current when a vehicle is stopped for a long time for such as shipment of cargo, it is thought to provide a protective device to turn the semiconductor switch composing the semiconductor fuse off to shut off the dark current after a lapse of a specific time after becoming the sleep mode (Patent document 3). However, according to this conventional protective device, it is necessary to maintain the semiconductor switch on and to get the peripheral circuit such as above-described current sensor and the controller to work normally until a specific time lapses after becoming the sleep mode. Therefore, there is a third problem that the power consumption on the peripheral circuit and the controller in the sleep mode cannot be reduced, and there is a fear that the battery may run out.    [Patent document 1] JP, A, 2007-203929    [Patent document 2] JP, A, 2002-78183    [Patent document 3] JP, A, H09-202195
Accordingly, a first object of the present invention is to provide a power feeding circuit configured to reduce the power consumption in the sleep mode. A second object of the present invention is to provide a power feeding circuit configured to reduce the number of semiconductor switches, power lines, and signal lines, and to save power. A third object of the present invention is to provide a power feeding device configured to reduce the power consumption in the sleep mode and to eliminate the dark current when a vehicle is stopped for a long time.